A linear 23-residue peptide reveals a propensity to form an unusual native-like conformation

To gain insight into the earliest events of protein folding, a 23-residue peptide with a sequence corresponding to the 38-60 fragment of the β-subunit of human chorionic gonadotropin (hCGβ) was studied by NMR. In aqueous solution the majority of the peptide residues adopted an extended polyproline II (Pn) conformation similar to those in mature, fully folded hCGβ. The finding that the isolated protein fragment may acquire native-like structural motifs, even without α-helices or β-structures, extends the possibility of using free peptides as model systems to better understand the protein folding mechanisms. It was shown that the P_II-rich structural motif can be determined efficiently by NMR spectroscopy. The observation that in the absence of extensive medium-and long-range interactions the majority of amino acid residues may adopt the P_IIconformation suggests that the P_II-rich structural motifs may play an important role in early events of protein folding.